In the field of heating water in domestic appliances a number of switch control systems are in use which include the use of temperature sensitive bimetallic strips to trigger a switch to open circuit an electrical supply to a heating element. Typically, the bimetallic strip is actuated by the temperature of steam rising from the boiling liquid which is directed to contact the bimetallic strip and thus trigger a switching mechanism.
It is also known to employ floats to trigger a switch to open circuit an electrical supply to a heating element to prevent, such as a kettle or urn, from boiling dry. The use of vibration sensors and ultrasonic wave reflection is also known in the control of apparatus for boiling water.
In contrast with these known arrangements the present invention employs the lifting force of bubbles which form in a boiling liquid to actuate a control system for applying thermal energy to the liquid.